In the first press conference he has given for six months, he defended US military action in Iran and Venezuela under his tenure.
The BBC's Chief North America correspondent Gary O'Donoghue breaks down what he said:
As we've reported, the effort is to impeach Hegseth is likely to fail in the short-term, as Republicans remain in power in Congress until January.
But if the House votes to impeach him, it will still be up to the chamber's leadership to decide when to send the matter to the Senate, where an impeached individual stands trial.
Impeachment resolutions can also be carried on past the end of a Congress, meaning that Democrats would be able to take it up next year when they are in the majority in both chambers of Congress.
Massie has acknowledged that the effort may not succeed immediately, but has argued that it might help push President Trump to replace Hegseth before Democrats can launch their own impeachment process.
The US president, however, has so far stood by his defence secretary.
Impeachment is a lengthy process. It starts in the House of Representatives and if it passes - which means more than half of politicians have voted in support of it - the official is impeached.
Then it moves to the Senate, where the official is put on trial and questioned by senators. Two-thirds of them must vote in favour of impeachment to remove the official from their job.
If the Senate doesn't convict them, the official remains in their job. For example, Donald Trump has previously been impeached twice, but was not convicted by the senate so remained in his post.
Democratic Congresswoman Delia C. Ramirez says in a post on X: "We have to impeach Hegseth. It is time to ACT."
Congressman Herb Conaway, also a Democrat, responds to a post with a video showing Hegseth saying: "If they want to come for the warriors, they are going to have to come through me" - more on that here.
He says: "Nobody is coming after our service members. We're coming after you for your reckless actions and dangerous incompetence."
Meanwhile, Republican Speaker of the House Mike Johnson tells reporters Thomas Massie is a "disgruntled congressman" and that the impeachment effort is a "political gambit for attention".
He says the idea that you impeach the secretary of war at a time of international conflict "sends a terrible message to our enemies and our allies".
And Republican Congressman Don Bacon says he will vote no to impeach Hegseth, despite being "not a fan of the secretary", because he says he doesn't agree with the impeachment itself, pointing to things he thinks are missing, including "how [Hegseth] has held money on Ukraine".